Process for manufacturing corrugated thermoplastic synthetic resin cardboard sheet

ABSTRACT

A continuous process for manufacturing with good reproducibility a corrugated thermoplastic synthetic resin cardboard sheet free of defects diminishing the commercial value of the produce such as &#39;&#39;&#39;&#39;warp&#39;&#39;&#39;&#39; and &#39;&#39;&#39;&#39;crookedness&#39;&#39;&#39;&#39;. This process is characterized in that thin layers of a thermoplastic synthetic resin having a melting point at least 10*C. lower than that of any of thermoplastic synthetic resins constituting the core sheet and surface liner sheets are formed on faces selected from the group consisting of (i) front and back faces of the continuous thermoplastic synthetic resin sheet to be shaped into a core sheet, (ii) faces to be bonded to the core sheet, of each of said surface liners, and (iii) all the faces mentioned in (i) and (ii), before the shaping of the core sheet and before the meltpress bonding of the surface liners onto the continuous core sheet.

United States Patent Kurita et a1. 1 51 Mar. 27, 1973 541 PROCESS FORMANUFACTURING 3,404,748 10/1968 Bjorksten ..156/2l0 x CORRUGATEDTHERMOPLASTIC 3,518,142 6/1970 Dooley l56/332 3,519,531 7 1970 James..156/332 gz ggg RESIN CARDBOARD 3,617,419 11 1971 Fischer ..156/332Inventors: Kiyoshi Kurita, Tokyo; Hiroshi Shigematsu, Ichihara; TomoyukiKoyama, Yokohama; Souichiro Endo, Tokyo; Kensaku Yamawaki, Ichihara, allof J apan Primary Examiner--Reuben Epstein Att0rney-Sherman andShalloway [57] ABSTRACT A continuous process for manufacturing with goodreproducibility a corrugated thermoplastic synthetic resin cardboardsheet free of defects diminishing the commercial value of the producesuch as warp and crookedness. This process is characterized in that thinlayers of a thermoplastic synthetic resin having a melting point atleast 10C. lower than that of any of thermoplastic synthetic resinsconstituting the core sheet and surface liner sheets are formed on facesselected from the group consisting of (i) front and back faces of thecontinuous thermoplastic synthetic resin sheet to be shaped into a coresheet, (ii) faces to be bonded to the core sheet, of each of saidsurface liners, and (iii) all the faces mentioned in (i) and (h), beforethe shaping of the core sheet and before the melt-press bonding of thesurface liners onto the continuous core sheet.

6 Claims, 7 Drawing Figures PATENIEUmzims SHEET 3 OF 4 Fig. 3

PATHHIUHARZYIHYS 3,7 3,222

SHEET u 0F 4 Fig. 4

PROCESS FOR MANUFACTURING CORRUGATED THERMOPLASTIC SYNTHETIC RESINCARDBOARD SHEET This invention relates to a continuous process formanufacturing with good reproducibility a corrugated thermoplasticsynthetic resin cardboard sheet free of defects diminishing thecommercial value of the product such as warp and crookedness. Moredetailedly, this invention relates to a continuous process formanufacturing a corrugated thermoplastic synthetic resin cardboard sheetcomprising continuously forwarding a continuous sheet of a thermoplasticsynthetic resin through a core sheet shaping zone to thereby shape therunning sheet into a core sheet of corrugated cardboard, and melt-pressbonding continuously surface liners of a continuous thermoplasticsynthetic resin continuous sheet, coincidentally or successively, ontofront and back faces of the shaped core sheet, such process beingcharacterized in that thin layers of a thermoplastic synthetic resinhaving a melting point at least C, lower than that the thermoplasticsynthetic resins constituting the core sheet and surface liner sheetsare formed on faces selected from the group consisting of i. front andback faces of the continuous thermoplastic synthetic resin sheet to beshaped into the core sheet,

ii. faces of each of the surface liners to be bonded to the core sheet,and

iii. all the faces mentioned in (i) and (ii), before the shaping of thecore sheet and before the melt-press bonding of the surface liners ontosaid continuous core sheet.

In the specification and claims, the term melting point" means Vicatsoftening point" determined by the method according to ASTM D 1525.

Since corrugated cardboard sheets manufactured from paper are poor inwater proofing property, corrugated cardboard sheets of water proofpaper which has been subjected to a water proof processing to improvethe poor water proofing property of the paper have been marketed, buttheir water proofing property is still insufficient.

Corrugated cardboard sheets manufactured from thermoplastic syntheticresins are as light and tough as those prepared from paper, and they arepacking materials very excellent in water proofing property.Accordingly, various proposals have been heretofore made to manufacturecorrugated cardboard sheets with use of thermoplastic synthetic resins.

According to such conventional techniques, corrugated cardboard sheetsare prepared by continuously forwarding a continuous sheet of athermoplastic synthetic resin through a core sheet shaping zone tothereby shape the running sheet into a core sheet of corrugatedcardboard, and melt-press bonding continuously surface liners of acontinuous thermoplastic synthetic resin sheet onto front and back facesof the shaped core sheet. Thus in the conventional methods it isnecessary that in the melt-press bonding of a continuous sheet forsurface layer onto a continuous sheet for core, both sheets must be in asoftened or molten state exhibiting a sufficient stickiness to give amelt-bonding between portions of both sheets to be pressed to eachother. Such melt-press bonding method is divided in two groups; thecoincidental melt-press bonding method comprising melt-press bondingsurface liners onto back and front faces of the continuous core sheetcoincidentally and the successive melt-press bonding method comprisingmelt-press bonding one surface liner onto one of faces of the continuouscore sheet and then melt-press bonding another surface liner onto theother face of the continuous core sheet.

In the coincidental method a continuous sheet which has been shaped intoa core sheet undergoes a pressure from both front and back faces duringthe pressing of surface liners thereon while the sheet is in thesoftened or molten state exhibiting a sufficient stickiness for bonding.Therefore, the configuration of the core sheet is easily deformed bysuch pressure, which results in decrease of the commercial value of theend product. In an extreme case, the product cannot be utilized ascorrugated cardboard. With a view to overcoming this disadvantage,complicated operations and controls have been conducted. For instance,the continuous sheet which has been shaped to have a core configurationis cooled to such a degree that the core configuration may resist thepressure caused by the liner pressing. In this case the cooling iseffected in a manner such that the convex top of the core configurationand its vicinity may not be cooled, or the convex top of the coreconfiguration and its vicinity are heated again to give a melt-bondablecondition thereto. Accordingly, the coincidental method necessitatessuch troublesome operations and temperature controls and expensiveadditional equipment are required for conducting such operations andcontrols, Further, even if corrugated cardboards are prepared by suchdisadvantageous method, it is impossible to obtain products ofsufficient quality with good reproducibility. Therefore, the preparationof corrugated cardboard sheets is now carried out by the successivemethod.

According to the successive method, while the continuous core sheet isin the softened state, a liner sheet which is also kept in the softenedstate is pressed and bonded to one face of the core sheet, and the coresheet having the liner on one face is naturally or positively cooled bya cooling bath or cool air sufficiently to maintain its coreconfiguration while it is forwarded to the next step. Thereafter,another liner sheet kept in the softened state is pressed and bonded tothe other face of the continuous core sheet. In this method, thedeformation of the core configuration can be avoided to a considerableextent, but the temperature difference between the primarily bondedsurface liner and the secondarily bonded surface liner is caused toappear and, therefore, the cooling contraction of the latter liner isgreater than that of the former liner, which results inevitably information of warp and crookedness.

The formation of warp" or crookedness" is disadvantageous in that in theprinting on the surface of the resulting corrugated cardboard product abeautiful, uniform printing cannot be obtained and that aftertreatmentswith a rotary slotter or slitter score are made difficult by thepresence ofwarp or crookedness and therefore, cases or boxes preparedfrom such corrugated cardboard sheets are extremely low with respect tothe commercial value.

In the successive method, as the difference of the contraction betweenfront and back liners is great, the formation of warp" or crookedness isextreme. As is well known in the art of film shaping, in the extrusionof a liner sheet the high take-up rate results in the great orientationin the stretch direction and in decrease of the commercial value of theproduct caused by a high contraction when the sheet is cooled. Forpreventing formation of warp or crookedness, the take-up rate in theextrusion of a liner sheet is limited below certain values in thesuccessive melt-press bonding method. Generally, the take-up rate shouldbe maintained below several tens of meters per minute. It isindustrially unprofitable to conduct the manufacture of corrugatedcardboard sheets at such low rates while using expensive corrugatedcardboard shaping machines.

With a view to developing a process for the manufacture of corrugatedcardboard sheets by which corrugated cardboard sheets of excellentquality may be prepared with good reproducibility and industrialadvantages and by which the above-mentioned defects of both thecoincidental and successive melt-press bonding methods may be overcome,we have made conducted research and now found that the abovementioneddefects of the conventional techniques can be conveniently overcome by aprocess inwhich thin layers of a thermoplastic synthetic resin having amelting point at least lC. lower than that of any of thermoplasticsynthetic resins constituting the core sheet and surface liner sheetsare formed on faces selected from the group consisting of i. front andback faces of the continuous thermoplastic synthetic resin sheet to beshaped into a core sheet,

ii. faces to be bonded to the core sheet, of each of surface liners, and

iii. all the faces mentioned in (i) and (ii), before the shaping of thecore sheet and before the melt-press bonding of the surface liners ontothe continuous core sheet.

Accordingly, the primary object of this invention is to provide aprocess which can manufacture continuously corrugated thermoplasticsynthetic resin cardboard sheets of excellent quality at high rates andwhich can overcome the defects and limitations of the conventionalmethods of preparing corrugated thermoplastic synthetic resin cardboardsheets and can give products free ofwarp" and crookedness.

Other objects and advantages of this invention will be apparent from thedescription given hereinbelow.

In the process of this invention, thin layers of a thermoplasticsynthetic resin (B) having a melting point at least C. lower than thatof any of thermoplastic synthetic resins (A) constituting the core sheetand surface liner sheets are formed on the above-mentioned faces (i),(ii) or (iii). In case thin layers are formed on faces of the coresheet, the formation of thin layers is effected before the shaping ofthe core sheet, namely before the sheet is shaped to have a coreconfiguration. In case thin layers are formed on the surface linersheets, the formation is effected before the liner sheets are melt-pressbonded to the core sheet.

The formation of such thin layers on the above-mentioned faces may beperformed by laminating or the like. In this invention it is preferableto use material sheets of resin (A) on which have been formed thinlayers of resin (B) in advance. However, the lamination of thin layersmay be conducted at any optional point as long as the abovementionedrequirement of laminating time is satisfied.

As the thermoplastic synthetic resin (A) constituting the core sheet orthe surface liner sheet, vinyl resins such as polyvinyl chloride,polyvinylidene chloride and polystyrene; and polyolefin resins such aslow density polyethylene, high density polyethylene, ethylene propylenecopolymer, polypropylene, polybutylene and poly-4-methylpentene-l may beconveniently used.

As the thermoplastic synthetic resin constituting the thin layer to beformed on the above faces (i), (ii) or (iii) resins may be preferablyused selected from the groups consisting of copolymers of ethylene witha saturated monobasic carboxylic acid vinyl ester, copolymers ofethylene with an unsaturated carboxylic acid, copolymers of ethylenewith an unsaturated carboxylic acid ester, terpolymers of ethylene witha saturated monocarboxylic acid vinyl ester and an unsaturatedcarboxylic acid, and partial metal salts of copolymers of ethylene withan unsaturated carboxylic acid. A suitable kind of the thermoplasticsynthetic resin (B) is selected depending on the kind of thethermoplastic synthetic resins (A) with the proviso that the meltingpoint of the resin (B) is at least 10C. lower than that of any of resins(A). A thermoplastic synthetic resin having a melting point of between50 and C. is preferably used as resin (B).

The thermoplastic synthetic resin (B) will be now detailed.

I. Copolymers of ethylene with a monobasic carboxylic acid vinyl ester:

As the saturated monobasic carboxylic acid vinyl ester, vinyl esters ofaliphatic carboxylic acids having 1 to 4 carbon atoms such as vinylformate, vinyl acetate, vinyl propionate and vinyl butyrate may be used.From the viewpoint of effectiveness and price the use of vinyl acetateis most preferred. A copolymer of ethylene with vinyl acetate containingat least 5 percent by weight of vinyl acetate is particularlypreferable. In some cases it is possible to use a partially saponifiedproduct of such ethylene-vinyl acetate copolymer.

2. Copolymers of ethylene with an unsaturated carboxylic acid or itsester:

As the unsaturated carboxylic acid acrylic acid, methacrylic acid,maleic acid, fumaric acid may be cited and itaconic acid. As the esterlower alkyl (C C esters such as methyl and ethyl esters are cited.Preferable acids and esters are acrylic acid, methacrylic acid, andmethyl and ethyl esters of these acids. Good results are obtainable bythe use of copolymers of ethylene with acrylic acid, methacrylic acid ora lower alkyl (C C ester thereof containing at least 5 percent by weightof such acid or ester.

3. Terpolymers of ethylene with a saturated monobasic carboxylic acidvinyl ester and an unsaturated carboxylic acid:

As the saturated monobasic carboxylic acid vinyl ester there vinylesters of lower aliphatic carboxylic acids having one to four carbonatoms such as vinyl formate, vinyl acetate, vinyl propionate and vinylbutyrate are preferably used. Vinyl acetate is used most adsaturatedvantageously. A preferred content of the vinyl ester in the terpolymeris 5 to 35 percent by weight.

As the unsaturated carboxylic acid monovalent unsaturated carboxylicacids such as acrylic acid and methacrylic acid; and methyl and ethylmonoesters of divalent unsaturated carboxylic acids such as maleic acid,fumaric acid and itaconic acid may be cited. Particularly good resultsare obtainable by the use of acrylic acid and methacrylic acid. Thecontent of such unsaturated carboxylic acid component in the terpolymeris generally in the range of from 0.01 to 1-0 percent by weight.

4. Partial metal salts of copolymers of ethylene with an unsaturatedcarboxylic acid:

There resins are disclosed in the specification of U.S. Pat. No.3,264,272. More specifically, such resins are prepared by reacting acopolymer of ethylene with an unsaturated carboxylic acid, with at leastone watersoluble ionic compound to thereby neutralize more than percentby weight of the carboxylic acid groups of the terpolymer.

Most preferable resins among above-mentioned resins l to 4 arecopolymers of ethylene with vinyl acetate having a vinyl acetate contentof at least 5 percent by weight.

It is preferable to incorporate into such polymers additives such asadhesion reinforcing agents, adhesion assistants, plasticizers andbulking fillers.

In the process of this invention, sheetsshaped from the aboveexemplified resins (A) by inflation method, T-die method, calendermethod, wet casting method or the like are used as the core and linersheets. As occasions demand, it is possible to use foamed sheetsprepared by employing an inorganic or organic foaming agent.

The thickness of the-sheet is not critical in this invention, but asheet of a thickness of 0.05 to 1.0 millimeter is usually selected asthe core or surface liner sheet.

The formation of thin layers of the thermoplastic synthetic resin (B) onfaces (i), (ii) or (iii) may be performed by customary laminatingmethods such as coextruding method and extrusion-coating method.

For better illustration of this invention, some embodiments of theprocess of this invention will now be detailed by referring to theappended drawings.

FIG. 1 is a diagrammatic side view showing an embodiment where theprocess is carried out according to the successive melt-press bondingprocedure.

FIG. 2 is a similar side view showing another embodi ment where theprocess is conducted according to the coincidental melt-press bondingprocedure.

FIG. 3 is a partially enlarged side view showing examples of thecorrugated cardboard sheet obtained according to the process of thisinvention.

In the successive method of melt-press bonding shown in FIG. 1, acontinuous thermoplastic synthetic resin sheet f to be shaped into acore sheet is forwarded from a delivery roll 1 and passed through a pairof embossing rolls 2, 2 by which the sheetf is shaped to have a coreconfiguration such as wavy, arching or saw-tooth configuration. Anoptional configuration is given to the sheet according to the shape ofembossments formed on the surfaces of embossing rolls. The embossingrolls are heated at a temperature suitable for facilitating theimpartion of the core configuration depending on the kind of the resinand the thickness of the sheetf If desired, it is possible to provide aheating device in the passage of the sheetf between roll 1 and embossingrolls 2, 2 to thereby heat in advance the sheet f to a temperature atwhich the impartion of the core configuration may be effected with ease.

A sheet f to be used as one of surface liners is forwarded from adelivery roll 3 and heated by a heating member 4, and is softened andmade sticky on the surface to be melted and press-bonded to the coresheetf which has been shaped to have a core configuration, to such anextent that adhesion may be attained between the two sheets when theyare pressed to each other by press roll 16. Thus sheetsf andf are meltedand pressbonded to each other by pressure of embossing roll 2, 2' andpress roll 16.

In this case, on the front and back faces of sheet f the faces to bebonded to the sheet f of the sheet f and sheet f which will be describedbelow, or all the faces mentioned above there are formed thin layers ofathermoplastic synthetic resin (B) having a melting point at least 10C.lower than that of any of thermoplastic synthetic resins (A)constituting core sheetf and surface liner sheets f and f;,. In theembodiment shown in FIG. 1 these thin layers are formed on each of thesheets in advance. If desired, it is possible to provide a member forlaminating resin (B) on running sheets at a suitable point betweendelivery roll 1 and embossing rolls 2, 2', or between delivery roll 3and press roll 16, or between delivery roll 11 and pinch roll 9'. Inview of facilitation of the operation and provision of equipment it isparticularly advantageous to use sheets on which thin layers of resin(B) have been formed in advance and further, when such sheets are used,corrugated cardboards of particularly excellent quality can be obtainedwith good reproducibility.

The formation of thin layers of resin (13) on core sheet f and surfaceliner sheets f and f may be effected by co-extruding methods such astwolayer-inflation method and two-layered T die method. It is alsopossible to conduct the formation of thin layers of low melting pointresin (B) by extrusion coating them on faces of sheets f f and f Stillfurther, thin layers of low melting point resin (13) may be formed bydry lamination according to the above-mentioned conventional methods.

Shaping of the resulting corrugated cardboard sheet can be accomplishedmore easily as the thickness of the thin layer of resin (B) is thicker.

From the economical viewpoint, however, it is desired to adjust thethickness of the thin layer of resin (B) to 0.01 to 0.10 millimeter.

A single facer composed of integrated core sheet f and surface linersheet f is continuously forwarded by a suitable member to a zone wherethe single facer is melted and press-bonded to another surface linerf;,. In the embodiment shown in FIG. 1, the delivery of the single faceris conducted by means of a guide roll 15, a conveyer belt 6 installedaround a suitable number of conveyer rolls 5, 5, 5" disposed at suitablepositions and guide rolls 7 and 8. Thus single facer is delivered topinch rolls 9, 9'.

Another surface liner sheet f is forwarded from delivery roll 11 towardpinch rolls 9, 9'. The sheetf is heated and softened only on the surfaceto be melted and press-bonded to the shaped core sheet at a suitablepoint during the passage from delivery roll 11 to pinch rolls 9, 9' by aheating member (not shown). In the embodiment shown in FIG. 1, thedelivery of the sheet f from delivery roll 11 to pinch rolls 9, 9' isperformed by an air support roll 12 and a guide roll 13.

Between pinch rolls 9, 9 the surface liner sheet f; is melted andpress-bonded to the exposed surface of the core sheet of the singlefacer. Thus a corrugated cardboard sheet is formed. The product sheet isheld between conveyer belts 14, 14 and continuously forwarded to theafter-treating step.

As is apparent from the above explanation made by referring to oneembodiment according to the successive melt-press bonding procedure, inthe process of this invention at the melt-press bonding it is unnecessary to heat and soften core sheet f or surface liner sheets f andfl, perse so as to make them sticky and melt-bendable, with the result that itis possible to maintain the thermal history of each sheet at anequivalent level. Accordingly, formation of warp or crookednesscaused bydifference of contraction in these sheets, which is inevitable in theconventional successive method of melt-press bonding, can be preventedand products of high commercial value can be prepared continuously withgood reproducibility. Further, the above-mentioned limitation imposed onthe take-up rate of the surface liner sheet in the conventionalsuccessive method can be excepted in the process of this invention.Namely, while in the conventional successive method the take-up rate ofsurface liner film is about 20 m/min at most, in the process of thisinvention the shaping can be carried out at a rate as high as 100 m/min.Furthermore, even when these material sheets, particularly core sheetfare cooled to low temperatures so as to fix the shaped configurationfirmly and sufficiently, it is enough that only thin layers of resin (B)are softened by heating to such an extent that only thin layers mayexhibit a stickiness at the melt-press bonding. In addition, themelt-ing point of resin (B) is at least 10C. lower than any of resins(A). Accordingly, there is no danger of deformation of the shapedconfiguration by pressure of pinch rolls 9, 9 at all.

FIG. 2 is a diagrammatic side view similar to FIG. 1, illustrating anembodiment of the process of this invention according to thecoincidental melt-press bonding procedure.

In FIG. 2, a sheetf to form a core is forwarded from a delivery roll 1and shaped to have a desired core configuration between embossing rolls2, 2' in the same manner as explained with respect to FIG. 1. The soshaped core sheet is continuously forwarded toward pinch rolls 9, 9'.The delivery of the core sheet may be performed by means of similarsuitable guide rolls and conveyer belt to those shown in FIG. 1, thoughthese means are not shown in FIG. 2. Further, as in the embodiment shownin FIG. 1, the shaped core sheet may be naturally or positively cooledduring the passage to pinch rolls to fix the core configurationsufficiently.

Two sheets f and f}, for surface liners are forwarded from deliveryrolls 3 and 1 1, respectively, and passed to pinch rolls 9, 9'. Duringthis passage the sheetsf andf may be heated on the surfaces to be meltedand pressbonded to the core sheet. In the Figure heating means 4, 4' areshown. It is possible to provide another heating means for heating thecore sheet or to provide, instead of heating means 4, 4, a duplexheating electric heater for heating both the core sheet and the surfaceliner sheets.

Between pinch rolls 9, 9 the surface liner sheetsf and j}, arecoincidentally melted and press-bonded to the surfaces of the shapedcore sheet f As is explained with respect to FIG. 1, since thin layersof low melting point resin (B) are formed on the faces (i), (ii) or(iii), in the process of this invention it is unnecessary to heat thesesheets to such an extent that these sheets per se exhibit stickiness.Consequently, deformation of the core configuration is not caused tooccur in the core sheet and for the reasons described with respect tothe embodiment of FIG. 1 occurrence of phenomenon of warp or crookednesswhich will decrease the commercial value of the product, can beprevented perfectly and since the process is carried out according tothe coincidental procedure, the prevention of formation of such defectsis more complete and products of more excellent quality can be obtainedwith good reproducibility.

In the embodiment shown in FIG. 2, preheaters 4", 4" are provided at thepoint before the resulting corrugated cardboard sheet is forwarded bydelivery conveyer belts 14, 14'. These preheaters are provided fordispersing and relaxing the stress which has been caused when the coreconfiguration was imparted to the core sheet and which remains in thecore sheet. Of course, these preheaters may be omitted.

Arrows in FIGS. 1 and 2 show the directions of movements of sheets androtations of rolls and conveyer belts.

Thus, in accordance with the process of this inven tion it is possibleto manufacture continuously corrugated cardboard sheets of excellentquality at improved manufacturing rates without any danger of formationof warp" or crookedness which will decrease the commercial value of theproduct. Although FIGS. 1 and 2 illustrate embodiments where core sheetsand surface liner sheets are forwarded after they have been once woundonto delivery rolls, it is also possible to combine the sheetpreparation step directly with the corrugated cardboard preparation stepand conduct both the steps continuously.

In FIG. 3a, FIG. 3b and FIG. 30 there are given partially enlarged sideviews of some examples of corrugated cardboard sheets manufacturedaccording to the process of this invention. FIG. 3a illustrates anexample where thin layers of low melting point thermoplastic syntheticresin (B) are formed on the faces bonded to the core sheet f1, of bothsurface liner sheets f and f5. FIG. 3b illustrates an example where thinlayers of low melting point resin (B) are formed on both the front andback faces of the core sheet f FIG. 30 illustrates an example where thinlayers of low melting point resin (B) are formed on the faces of thecore sheet f and the faces bonded to the core sheet f of surface linersheets f and f5.

Typical working examples of the process of this in vention will now bedescribed below with reference to either the successive melt-pressbonding procedure or coincidental melt-press bonding procedure. Partsand percentages in examples are based on the weight unless otherwiseindicated.

Example 1 A sheet f to be formed in a core to be used in this Example isa sheet of 0.3 mm thickness shaped from a dry blend resin of 8 parts oflow density polyethylene and 2 parts of ordinary polystyrene. Each ofsurface liner sheets f and f is prepared by extrusion coating a 0.02 mmthick lamination of a copolymer of ethylene and vinyl acetate on onesurface of a 0.4 mm substrate composed of a foamed sheet obtained byfoaming at a foaming ratio of 1.8 a mixture of a dry blend resin of 7parts of high density polyethylene and 3 parts of lowdensitypolyethylene and 0.05 part of a azodicarbonamide (foaming agent). Themelting point of the above ethylenevinyl acetate copolymer is 64C. Themelting point of the dry blend resin constituting the core sheet is90C., and that of the dry blend resin constituting the surface linersheet is 89C.

The Example is carried out according to the embodiment shown in FIG. 1.

The above-mentioned core sheet f is forwarded from a delivery roll 1 andpassed through embossing rolls 2,2 heated at 95C., to impart a coreconfiguration to the sheet f One surface liner sheet f is forwarded froma delivery roll 3 and its laminate layer of the copolymer ofethylenevinyl acetate is heated by a preheater 4. Then, the shaped coresheetf and the surface liner sheet f are melted and press-bonded to eachother by a press roll 16. At the melt-press bonding, the temperature ofthe ethylene-vinyl acetate copolymer was 70C., and temperature of thesubstratum of the sheet f is 50C. The single facer, on one of whosesurfaces the surface liner sheet has been bonded is allowed to pass aguide roll and a conveyer roll 6, and then moves through guide rolls 7and 8 at a rate of 60 m/min. Another surface liner sheet f similar tothe sheet f is forwarded from a delivery roll 11, and the laminate layerof the sheetf is heated at 70C. and the substratum thereof is heated at50C., while being supported by an air support roll 12 then, the sheetfis forwarded through a guide roll 13 toward pinch rolls 9,9. Betweenpinch rolls 9, 9' the surface liner sheetf forwarded from the deliveryroll 11 is melted and pressbonded to the single facer forwarded from theguide roll 8.

The so formed corrugated cardboard sheet material is forwarded to theaftertreatment step by means of conveyers 14, 14.

Com-

Physical properties of the so prepared corrugated cardboard sheet areshown in Table 1 below.

For comparison, a corrugated cardboard sheet is prepared in the samemanner as above, except that thin laminate layers of a low melting pointresin are not formed on any of the faces of the core sheetf and surfaceliner sheets f and f and that heating means are provided just beforepinch rolls 9, 9 to heat the sheets to such an extent that they aresticky and bondable to each other (Comparative Example 1). Physicalproperties of the comparative product are also shown in Table Table 1Product of Com- Product of Example 1 parative Example I Feel strength(kg/in) 0.39 0.42 Crush strength 7.09 7.25

Values of peel strength, crush strength and ratio of formation of warp"are determined and evaluated by the following test methods.

1. Peel strength The testing is effected in the light of the test for T-peel for adhesive" according to ASTM D l87669.

Five sheets of test pieces shown in FIG. 4 as shown in the testingpublication are prepared. Both ends of each test piece are stretched tothe direction shown by the arrow in the drawing at a constant rate of 15mm/min by means of a tensile tester, and the maximum load which appearswhile the piece is completely peeled, is read. The mean values ofreading of the load of five test pieces is defined as peel strength. Thestretching direction is vertical to the bonding line direction. Otherconditions such as humidity and room temperature are based on ASTM D1876-69. (In the drawing, q 1 inch, r 9 inches, s 3 feet and t 3inches.)

2. Crush strength The testing is effected in the light of the standardmethod of test for ring crush of paper board according to ASTM D ll6460.

Five sheets of test pieces shown in FIG. 5 are prepared and each testpiece is compressed at a rate of 15 mm/min in the same direction as thedirection of the bonding line. The maximum value of the load, whichappears while the test piece is completely crushed, is

' read, and the load is divided by the pressure-receiving length (I inthe drawing). A means of the so obtained values of five test pieces isdefined as crush strength. (In the drawing, 1 mm, m 50 mm, m 5 mm and p8 mm.)

3. Ratio of formation of warp Five pieces shown in FIG. 5 as shown inthe testing publication are cut from optional portions of the resultingcorrugated cardboard sheet and fixed on the surface of a wooden plate bymeans of an adhesive. A hard rubber roll on which a printing ink hasbeen thinly applied is pressed on the test pieces in the direction ofthe bonding line to transfer the ink of the roll onto the surfaces ofthe test pieces.

With respect to each test piece, the value A is calculated by thefollowing formula:

A (Surface area of the test piece on which the ink is not applied/Wholesurface area of the test piece) X 100 A mean of values A of the fivetest pieces is defined as ratio of formation of warp of the testedcorrugated cardboard sheet.

Example 2 A sheetf for core to be used in this Example is a 0.3 mm thicksheet prepared by shaping a blend resin composed'of 6 parts of polyvinylchloride and 4 parts of ordinary polystyrene. Each of surface linersheets f and f is a two-layer inflation film composed of a substratumsheet of high density polyethylene and a laminate sheet of a terpolymerof ethylene with 28 percent of vinyl acetate and 1 percent of ethylmethacrylate. The thickness of the substratum sheet is 0.2 mm and thatof the laminate sheet is 0.02 mm. The melting point of the blend resinof the core sheet f is 83C. and that of the Table 4 Peel strength(kg/in) 0.12 Crush strength (kg.cm) 4.25 Ratio of formation of warp"(96) 5.5

What we claim is:

l. A continuous process for the production of a corrugated fiber-freethermoplastic synthetic resin sheet which consists essentially ofcontinuously forwarding a continuous sheet of a thermoplastic syntheticresin (A) selected from the group consisting of polyvinyl chloride,polyvinylidene, chloride, polystyrene, low density polyethylene, highdensity polyethylene, ethylene-propylene copolymer, polypropylene,polybutylene and poly-4-methylpentene-l front and back surfaces of whichare coated with thin layers of a copolymer (B) of ethylene and vinylacetate containing at least 5 percent vinyl acetate, said copolymer (B)having a melting point at least C. lower than thermoplastic resin (A),to a core sheet shaping zone to shape said sheet of thermoplastic resin(A) into a corrugated core sheet; forwarding said corrugated core sheetto a pre-heating zone to soften said thin layers of copolymer (B) coatedon the front and back surfaces of said corrugated core sheet; forwardingsaid corrugated core sheet from said pre-heating zone to a melt-pressbonding zone; simultaneously forwarding surface liners of a continuousthermoplastic resin (A) selected from the group consisting of polyvinylchloride, polyvinylidene chloride, polystyrene, low densitypolyethylene, high density polyethylene, ethylene-propylene copolymer,polypropylene, polybutylene and poly-4- methylpentene-l to saidmelt-press bonding zone in a manner so that said surface liners aremelt-press bonded to opposite surfaces of said corrugated core sheet;and withdrawing said fiber-free corrugated thermoplastic synthetic resinsheet from said melt-press bonding zone.

2. The process of claim 1 wherein said corrugated core sheet and surfaceliner sheets have a thickness of from 0.05 to 1.0 millimeter.

3. The-process of claim 1 wherein the thickness of the thin layer ofcopolymer (B) is within a range of from 0.01 to 0.1 millimeter.

4. A continuous process for the production of a corrugated fiber-freethermoplastic synthetic resin sheet which consists essentially ofcontinuously forwarding a continuous sheet of a thermoplastic syntheticresin (A) selected from the group consisting of polyvinyl each sheetbeing coated with a thin layer of a copolymer (B) of ethylene and vinylacetate containing atleast S percent vinyl acetate, said copolymer (B)having a melting point at least 10C. lower than thermoplastic resin (A)to a pre-heating zone to soften said thin layers of copolymer (B);simultaneously forwarding said corrugated core sheet and said surfaceliners to a melt-press bonding zone in a manner so that said surfaceliners are melt-press bonded to said core sheet with the softened thinlayers of copolymer (B) in contact with said corrugated core sheet; andwithdrawing said fiber-free corrugated thermoplastic synthetic resinsheet from said melt-press bonding zone.

5. The process of claim 4 wherein said corrugated core sheet and surfaceliner sheets have a thickness of from 0.05 to 1.0 millimeter.

6. The process of claim 4 wherein the thickness of the thin layer ofcopolymer (B) is within a range of from 0.01 to 0.1 millimeter.

2. The process of claim 1 wherein said corrugated core sheet and surfaceliner sheets have a thickness of from 0.05 to 1.0 millimeter.
 3. Theprocess of claim 1 wherein the thickness of the thin layer of copolymer(B) is within a range of from 0.01 to 0.1 millimeter.
 4. A continuousprocess for the production of a corrugated fiber-free thermoplasticsynthetic resin sheet which consists essentially of continuouslyforwarding a continuous sheet of a thermoplastic synthetic resin (A)selected from the group consisting of polyvinyl chloride, polyvinylidenechloride, polystyrene, low density polyethylene, high densitypolyethylene, ethylene-propylene copolymer, polypropylene, polybutyleneand poly-4-methylpentene-1 to a core sheet shaping zone to shape saidsheet of thermoplastic resin (A) into a corrugated core sheet;continuously forwarding surface liner sheets of a thermoplastic resin(A) selected from the group consisting of polyvinyl chloride,polyvinylidene chloride, polystyrene, low density polyethylene, highdensity polyethylene, ethylene-propylene copolymer, polypropylene,polybutylene and poly-4-methylpentene-1, one surface of each sheet beingcoated with a thin layer of a copolymer (B) of ethylene and vinylacetate containing at least 5 percent vinyl acetate, said copolymer (B)having a melting point at least 10*C. lower than thermoplastic resin (A)to a pre-heating zone to soften said thin layers of copolymer (B);simultaneously forwarding said corrugated core sheet and said surfaceliners to a melt-press bonding zone in a manner so that said surfaceliners are melt-press bonded to said core sheet with the softened thinlayers of copolymer (B) in contact with said corrugated core sheet; andwithdrawing said fiber-free corrugated thermoplastic synthetic resinsheet from said melt-press bonding zone.
 5. The process of claim 4wherein said corrugated core sheet and surface liner sheets have athickness of from 0.05 to 1.0 millimeter.
 6. The process of claim 4wherein the thickness of the thin layer of copolymer (B) is within arange of from 0.01 to 0.1 millimeter.